1. Field of the Invention
The present invention relates to a novel heat resisting steel which is made of a Cr—Mo—V low-alloy steel and has superior high-temperature strength and superior anti-creep brittleness and which is used for rotor shafts of high-pressure, intermediate-pressure and high/intermediate-pressure steam turbines. The present invention also relates to a rotor shaft using the heat resisting steel, a steam turbine using the rotor shaft, and a power plant using the steam turbine.
2. Description of the Related Art In general, a Cr—Mo—V low-alloy steel according to ASTM standards (Designation: A470 class 8) is employed for high-pressure, intermediate-pressure and high/intermediate-pressure turbine rotors which are subjected to steam at high temperatures (steam temperatures of 538 to 566° C.). Recently, an improvement in power generation efficiency of steam turbines has been demanded from the viewpoint of energy saving, and an increase of steam temperature in a thermal power plant has been put into practice because increasing the steam temperature and pressure is a most effective measure to improve the power generation efficiency. At high temperatures in the range of steam temperatures of 566 to 600° C. including ultra super critical pressure, a 12%-Cr steel having high durable temperature and superior anti-environment characteristics is employed. Power generation at higher efficiency can save fossil fuel, reduce the amount of exhaust gases generated, and contribute to protecting global environments.
Patent Reference 1; JP,A 10-183294 discloses a heat resisting steel made of a Cr—Mo—V low-alloy steel, which contains 0.15-0.40% by weight of C, not more than 0.1% of Si, 0.05-0.25% of Mn, 1.5-2.5% of Ni, 0.8-2.5% of Cr, 0.8-2.5% of Mo and 0.15-0.35% of V, and which has a (Mn/Ni) ratio of not more than 0.12, i.e., a (Ni/Mn) ratio of not less than 8.3, and also discloses a high/low-pressure integral steam turbine employing the heat resisting steel for a rotor shaft.
Patent Reference 2; JP,A 9-41076 discloses that a Cr—Mo—V low-alloy steel containing 0.1-0.3% by weight of C, not more than 0.05% of Si, not more than 0.1% of Mn, 0.1-1.5% of Ni, 0.5-3% of Cr, 0.05-0.5% of Mo, 0.1-0.35% of V, 0.01-0.15% of Nb, 0.5-2% of W, and 0.001-0.01% of B is employed in a high/low-pressure integral steam turbine, i.e., as materials of rotors on the high- and low-pressure sides of the steam turbine.
Patent Reference 3; JP,A 9-194987 discloses that a Cr—Mo—V low-alloy steel containing 0.05-0.15% by weight of C, 0.005-0.3% of Si, 0.01-1.0% of Mn, 0.1-2.0% of Ni, 0.8-1.5% of Cr, 0.1-1.5% of Mo, 0.05-0.3% of V, and 0.1-2.5% of W is employed for a high-temperature rotor of a steam turbine.
Patent Reference 4; JP,A 9-268343 discloses that a Cr—Mo—V low-alloy steel containing 0.05-0.30% by weight of C, 0.005-0.3% of Si, 0.01-1.0% of Mn, 0.1-2.0% of Ni, 0.8-3.5% of Cr, 0.1-2.5% of Mo, 0.05-0.4% of V, and 0.1-3.5% of Co is employed for a high-temperature rotor of a steam turbine.